This invention relates to the field of alloys and more particularly to certain alloys which are resistant to corrosion in contaminated or uncontaminated sulfuric acid over a wide range of acid strengths.
For purposes of analyzing and predicting their corrosive effect on various metals, acids and other corrosive agents are commonly classified as either "oxidizing" or "reducing". A reducing medium is one which includes no component more oxidizing than the hydrogen ion or hydronium ion. Sulfuric acid is normally a reducing medium, as are hydrochloric acid, acetic acid, phosphoric acid, aluminum chloride, hydrobromic acid and hydrofluoric acid. Oxidizing media are those which include a component which is more oxidizing than the hydrogen ion or hydronium ion. Typical oxidizing media include nitric acid, hydrogen peroxide, ferric sulfate, silver nitrate, potassium nitrate, sodium nitrate, copper sulfate, potassium permanganate, sodium dichromate, chromic acid, calcium chloride, mercuric chloride, aqua regia, sodium hypochlorite, ferric chloride and cupric chloride.
The corrosion resistance of a given metal or alloy in a reducing medium is often sharply different from its resistance in an oxidizing medium, with some metals and alloys being more resistant to reducing media and others to oxidizing media. These differences in behavior are thought to be attributable to differences between the corrosion mechanism in reducing media and the corrosion mechanism in oxidizing media. Thus, corrosive attack by a reducing acid is generally considered to involve attack on the metal by hydrogen ions, resulting in oxidation of the metal to soluble ions and release of hydrogen gas. Thus, metals of relatively high nobility, as indicated by their positions in the galvanic series, are generally resistant to corrosion by reducing acids. Attack by oxidizing media, on the other hand, does not involve release of hydrogen but commonly results in the formation of metal oxides or other metallic compounds at the metal surface. Unlike the usual situation with reducing acids, a favorable position relative to hydrogen in the electromotive series provides no insurance that a metal will not be rapidly attacked by oxidizing media. However, certain elements such as chromium, aluminum and silicon form tough insoluble oxide films on initial contact with an oxidizing medium which serve as barriers against further reaction between the medium and the metal and thus prevent further corrosion from taking place.
These are a number of relatively expensive metals, for example, tungsten and tantalum, which exhibit a high resistance to corrosion in either a reducing or an oxidizing medium. However, many of the metals which are more commonly used for corrosion resistance in the reducing media, such as molybdenum and copper, are not particularly resistant to oxidizing media while the metals which are commonly used to impart resistance to oxidizing media, such as chromium, are rapidly attacked in various reducing media. For these reasons, many industrial alloys are designed for use in only one type of medium and often fail catastrophically when exposed to the other type.
Sulfuric acid is normally a reducing acid. High strength sulfuric acid, however, is often oxidizing, especially at elevated temperatures. Moreover, various industrial sulfuric acid streams contain various oxidizing acids and salts as contaminants. It is therefore desirable that an alloy designed for general utility in industrial sulfuric acid streams by resistant to both reducing and moderately oxidizing environments. To develop an alloy which is resistant to corrosion in sulfuric acid over a wide range of concentrations, however, it is not sufficient to provide resistance under the reducing conditions presented by dilute acid and the oxidizing conditions presented by concentrated acid. For many alloys, acids in the intermediate concentration range of 30-80% are more corrosive than either very concentrated or very dilute acid. The range of 60-70%, which is quite commonly encountered in various industrial processes, often results in particularly high rates of corrosion. Few commercial alloys are available which exhibit high resistance throughout the middle range of acid concentration and at the ends of the concentration range as well.
Commercially available alloys which have found use in sulfuric acid service are mostly nickel base alloys containing varying amounts of chromium, molybdenum, copper, silicon and manganese. As is generally the case in alloying practice, these alloys also contain the maximum amount of iron consistent with retention of reasonably good anticorrosive properties. In most of the alloy formulations which have been heretofore considered suitable for sulfuric acid service, particularly under severe conditions, the maximum allowable iron content has not been high. The sum of proportions of "critical elements" such as nickel, chromium, molybdenum and copper has normally been too great to allow the use of ferro alloys in alloy formulation. Those few sulfuric acid-resistant alloys whose critical metal content has been relatively low have typically included at least 18% by weight chromium. An exception to this general proposition is found in the existence of various iron/silicon alloys such as that sold under the trade designation "Duriron" by the Duriron Company. However, iron/silicon alloys, while possessed of a relatively high degree of corrosion resistance, are hard and brittle and cannot generally be cold-worked.
It is also well-known that, in a particular corrodent, the presence of contaminating chlorides, especially hydrochloric acid, will severly depassivate an alloy that is otherwise normally resistant to that corrodent.
Those skilled in the art of developing and applying alloys in the field of corrosion have for years been aware of the fact that an element which is beneficial to an alloy in one type of service is often detrimental in another type of service. Consequently, a very careful selection and balance of the proportions of elements is required to provide an alloy resistant to different corrosive environments such as those encountered in sulfuric acid solutions at different concentrations and temperatures.